The present invention relates to polarisation image detection systems.
Most of the light that reaches our eyes or man-made sensors comes indirectly through scattering. An object is usually viewed by observing the scattered light that is reflected from the object's surface. Scattering is the observable interaction between light and matter and occurs at all wavelengths in the electromagnetic spectrum. These scattering processes are fundamental to the production of polarisation in the universe.
When light or any form of electromagnetic radiation is reflected from a surface it may become polarised, the type and degree of polarisation being a function of angle of incidence, surface structure/texture and material type. Certain materials can also alter the polarisation state of electromagnetic radiation passing through them. These materials are known as optically active materials.
A polarisation image detector forms an image signal indicating variations in optical polarisation of light from a scene. When the signal is reproduced as a picture on a television screen, it illustrates surface structure of the scene rather than reflected intensity or colour. Polarisation image detectors may be used in a number of fields, for example in microscopy, medical diagnosis, meteorology and so on.
A known polarisation image detector described in British Patent Specification 1472854 comprises two television camera tubes which receive light via a beam splitter comprising a partially reflecting mirror and via respective polarisers of which the polarising directions are set at right angles to one another. After gamma correction to compensate for non-linearities of the camera system, the video signals are combined to form a polarisation representative picture signal.
In another known form of polarisation image detector, instead of two camera tubes, there can be used a single tube having two separate photo-sensitive screens or there can be used a single camera with a single screen in association with a polariser, for example a rotating polarising plate or an electro-optic crystal, operable to change periodically the polarisation direction of the light received by the camera, and with signal delay means operable to bring the signals obtained during the respective periods into synchronism.
In our earlier patent application No. 8125282, we describe a polarising image detector comprising polarising means for receiving optical radiation from a scene and for forming images of the scene respectively constituted by radiation components which differ in respect of the polarisation thereof, two charge-coupled area imaging devices for forming electrical picture signals corresponding to said images, and means for combining said picture signals to form a signal containing information about the polarisation of the optical radiation from the scene.
One example of our earlier invention was described in which the means for receiving optical radiation from the scene and for forming separate images with different polarisation comprised a Wollaston block double image prism and lens arrangement. This arrangement split the partially polarised light received from the scene into two orthogonally polarised images which are focussed onto two separate charge-coupled devices (CCDs). The electrical picture signals produced by these CCDs are summed and subtracted, and a signal representing the division of their subtraction by their sum is applied to a television receiver or stored on a video recorder or further processed by a data processor to give structural information concerning every point on the scene as opposed to colour or brightness information produced by a conventional television arrangement.
In other examples described in our earlier patent application, the Wollaston block is replaced by a Glan-Thompson prism. Various alternative arrangements are described in which, for example, the electrical picture signals produced by the charge-coupled devices are digitised by analogue-to-digital converters and processed by computer to form the required polarisation-dependent image data. The advantage of the latter arrangement is that known digital techniques for image enhancement or recognition could then be applied to the data to improve the quality of the polarisation-dependent image data extracted. These earlier systems are only capable of resolving linear polarisation forms.
It is an objective of the present invention to provide a polarisation image detection system which can resolve all forms of polarisation information emanating from a scene or object under examination. The new system can produce three different types of image representing the polarisation factor, degree of polarisation and polarisation phase angle for elliptically polarised light. It should be noted that elliptical polarisation includes all polaristaion types; linear and circular polarisation forms are just two specific cases of elliptical polarisation.